For example, in a belt for auxiliary machine drive of an automobile engine, belt tension varies due to rotation fluctuation caused by engine combustion. A belt slip occurs due to such variation in belt tension, and thus a problem such as slip sound or belt wear occurs. In order to solve this, conventionally, an auto-tensioner has been adopted as a mechanism to suppress the occurrence of belt slip even if belt tension varies.
For example, an auto-tensioner of Patent Document 1 is provided with: a base having a first cylindrical part; a rotating member which has a second cylindrical part that is disposed inside of the first cylindrical part and is supported so as to be rotatable with respect to the base, and on which a pulley around which a belt is wound can be mounted; a coil spring which is disposed inside of the second cylindrical part and rotationally biases the rotating member in one direction with respect to the base; and a friction member which is disposed between the first cylindrical part and the second cylindrical part, is slidable on the inner circumferential surface of the first cylindrical part, and has a convex portion which is engaged with a concave portion provided in the outer circumferential surface of the second cylindrical part.
Further, an auto-tensioner of Patent Document 2 is provided with: a base; a rotating member supported so as to be rotatable with respect to the base; a coil spring which rotationally biases the rotating member in one direction with respect to the base; a friction member which is disposed between the inner circumferential surface of a cylindrical part provided in the rotating member (or the base) and the coil spring and is slidable on the inner circumferential surface of the cylindrical part; and a leaf spring which is coupled to the friction member and has one end sandwiched between the rotating member (or the base) and an end portion of the coil spring in a circumferential direction.
In the auto-tensioner as in Patent Documents 1 and 2, between the case where belt tension increases and the case where it decreases, the magnitudes of a frictional force which is generated at a sliding surface of the friction member are different from each other, and thus a damping characteristic asymmetrical depending on a rotational direction of the rotating member (asymmetric damping characteristic) is achieved. That is, in the case where belt tension increases, by generating a large frictional force, it is possible to sufficiently damp the oscillation of the rotating member, and in the case where belt tension decreases, a small frictional force is generated, and therefore, it is possible to make the rotating member follow tension fluctuation of the belt.